A technique by which transistors are formed using semiconductor thin films formed over a substrate having an insulating surface has been attracting attention. Such transistors are applied to a wide range of electronic devices such as integrated circuits (IC) and image display devices (also simply referred to as display devices). A silicon-based semiconductor material is widely known as a material for a semiconductor thin film applicable to a transistor. As another material, an oxide semiconductor has been attracting attention.
For example, a technique for forming a transistor using zinc oxide or an In—Ga—Zn—based oxide semiconductor as an oxide semiconductor is disclosed (see Patent Document 1).